With increasingly slimed down, more multifunctional electronic devices, much higher densities are now in demand for circuit boards used therewith, too.
Among known approaches to enhancing the density of circuit boards, there is a method for fabricating a circuit board in a multilayered form. The multilayered circuit board (also referred to as a multilayer circuit board) may usually be obtained by a process of laminating a second electrical insulation layer (2) on an internal layer substrate comprising an electrical insulation layer (1) and a conductor circuit (a) formed on its surface and forming a second conductor circuit (b) on the second electrical insulation layer (2), and repeating that process as many times as necessary if required, whereby given alternations of insulation layers and conductor circuits are obtained.
Application of high-density wires to electronic equipment of multilayer structure causes its board or electronic device to generate heat as such during operation. To fend off setting fire by heat generation, flame retardants are usually incorporated in the electrical insulation layers. For the flame retardants, those insoluble in organic solvents are widely used. Among those flame retardants, preference is given to halogen-free flame retardants such as salts of basic nitrogenous compounds and phosphoric acid in consideration of environments.
For instance, the inventors have come up with a curable resin composition that contains an insulation resin and halogen-free flame retardant particles having a primary particle average major axis of 0.01 to 5 μm, an aspect ratio of up to 5 and the proportion of particles having a major axis of greater than 10 μm being at most 10% by number (JP(A)2002-121394). Such halogen-free flame retardant particles, for instance, may be prepared by wet milling of a salt of a basic nitrogenous compound and phosphoric acid in a mixed solvent comprising a polar solvent and a non-polar solvent. The curable resin composition is used as a varnish while an insulting resin and halogen-free flame retardant particles are dissolved or dispersed in an organic solvent.